Variable speed ratio traction drives are well known and have been used for years in applications where efficiency and the power to weight ratio was not of primary concern. Up to the present time, due to their multiple power paths, the most successful of the commercially available traction drives have been of the planetary type. However, even these are high weight, low power units. The inherent advantages of using rollers rather than gears to transmit power, such as smooth, vibration free power transfer, is offset by the limited capability of traction drives to transmit any appreciable tangential force through the traction engagement area. This is due to the very low coefficient of traction of even specially formulated traction lubricants. In addition to the very low coefficient of traction, rotational shearing of the lubricant within the traction engagement area is considered by those knowledgeable in the field, to be the single largest deterrent to high performance in traction drives. The heat generated by this shearing action can be substantial. U.S. Pat. No. 2,020,677, U.S. Pat. No. 3,099,927 and U.S. Pat. No. 4,369,667 demonstrate the persistence of this problem. The geometry of all existing traction drives allows very little flexibility in the size or shape of its traction engagement area. The elastic deflection of the mating traction elements define the geometry of the engagement area. The length of the engagement area, in the rolling direction, is predetermined by the rolling radius of the mating elements. Thus, the majority of traction drives in use today, have engagement area ellipses in which the width of the engagement area (transverse to the rolling direction) is up to four times the length. It is the width of the engagement area which causes spin related oil shear and the great loss of efficiency. The fact that many conventional traction drives use excessive load, normal to the engagement area, is further evidence of the efforts being made to increase the capacity of these traction drives without further increasing the width of the engagement area and thus, lubricant shear.
While some of these disadvantages are overcome in U.S. Pat. No. 4,524,642, it should be observed that the present invention has fewer rotating elements and a shorter power path consisting of only two engagement areas.